Electron Cloud Studies for Tevatron and Main Injector

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Electron Cloud Studies for Tevatron and Main
Injector

• Xiaolong Zhang
• AD/Tevatron

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In This Report

• Whats Electron Cloud and its Effects
• The impact on Main Injector Upgrades and the
  research activities at Accelerator Division
• Simulation methods, programs and results
• Future study plans.

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Mechanism of Electron Cloud Buildup

• Short bunch
• Initial electron produced by photos, beam loss,
  ionization, etc.
• Density of the electron increased by generating
  secondary electrons.
• Exponential growth of electron density happens
  with appropriate beam conditions.
• Electron cloud saturated by its space charge
  effect.

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Electrons Trapped in the Beam

• Long bunch or coasting beam
• Initial electron generated
• Electrons are trapped by beam potential
• Trailing edge multipacting (long bunch case)

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Effects of Electron Cloud

• Vacuum instabilities
• Fast vacuum jumps of several order of magnitude
• Beam instabilities
• Beam losses
• Heat loading
• Noise on beam instruments

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History of Electron Cloud Studies

• 1967 Novosibirk proton rings with coasting beam
• Two Stream Beam Instabilities
• Cure various beam intensity and clearing
  electrode
• 1970 CERN ISR coasting beam
• Cure clearing electrode
• 1977 ISR bunched beam.
• Vacuum pressure jumps
• End of 80s
• KEK PF Beam instabilities when switched from
  electron to positron
• PSR Beam instabilities
• 1995 Two B-Factories
• KEKB Simulation code PEI Beam studies KEK-BEPC
  
• PEPII Simulation code POSINST LBNL-SLAC TiN
  coating
• 1997 ECLOUD for LHC and SPS.
• CESR, APS, SNS, RHIC, etc.
• New simulation codes and methods keep appearing.
• Extensive SEY measurements material and surface
  treatments.

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Electron Cloud for Various Accelerators
MI
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Effects of the Electron CloudBeam Instabilities
KEKB
Sideband Peak Height
BEPC
PSR, 1988
Betatron Oscillation Sidebands
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Effects of the Electron CloudBeam Emittance
Growth
SPS
KEKB
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Effects of the Electron CloudVacuum Pressure Bump
RHIC
SPS
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Effects of the Electron CloudNoise on BPM Pickup
SPS
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Effects of the Electron CloudBeam Loss
RHIC
SPS
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Effects of the Electron CloudEstimation of the
Heat Load for LHC (Frank Zimmermann)
arc heat load vs. intensity, 25 ns spacing,
best model
R0.5
ECLOUD simulation
dmax1.7
dmax1.5
BS cooling capacity
injection
low luminosity
dmax1.3
high luminosity
dmax1.1
dmax1.3-1.4 suffices
calculation for 1 train
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Mitigations (1)

• Beam Scrubbing

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Mitigations (2)

• Bunched Beam Injection Pattern
• Solenoid

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Mitigations (3)

• Surface Coating with TiN or TiZrV (NEG)
• Surface Grooving

1 mm
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Mitigations (4)

• Clearing Electrode

E
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Simulations

• Small section of the beam pipe.
• Macro particles and discrete beam kick
• Space charge, electron and beam image charge
  included.
• Gaussian bunches (longitudinal bunch profile
  available for long bunches)
• Realistic secondary electron yield model.
• Electron longitudinal motion neglected
• Theoretical primary electron distributions

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Secondary Electron Yield
SLAC
CERN
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Activities at Accelerator Division

• Initial observation of pressure rise at Tevatron
  with high intensity uncoalesced beam in Dec.
  2002.
• Initiated by Weiren Chou and Francois Ostiguy for
  Proton Driver Study in April 2005.
• More beam studies at Tevatron and some
  observations at Main Injector
• Obtained simulation codes POSINST, ECLOUD, PEI,
  etc.
• Collaborations with LBNL, CERN, APS, BNL, SLAC,
  etc.
• Got 2 RFA electron detector as gifts from APS

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RFA Testing Beam Pipe in Tevatron and MI
RFA
ION GAUGE
ION PUMP
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Beam Studies at Tevatron (1)
Bunch intensity threshold around 4e10/bunch for
30 bunches, vacuum worsen _at_warm section A0, D0,
C0, E0, not _at_B0 and F0
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Beam Studies at Tevatron (2)
Tevatron 150GeV, 116e10/30bunches
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Beam Studies at Tevatron (3)
Some beam Schottky power rise observed when the
vacuum pressure rising
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Simulations for MI Upgrades

• Basic beam parameters

Beam energy 8.9 GeV
Ring circumference 3319.419 m
Maximum bunch intensity 30e10/bunch
Bunch number 6 batch of 84 bunches
Bunch spacing 5.645 m
Maximum bunch length 0.75 m Gaussian
Beam size rms 5 mm round
Residual gas pressure 20 nTorr, room tempeture
Beam pipe 6.15 cm x 2.15 cm elliptical
Electrons/proton loss 1.27e-7 (e/p)/m
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Elliptical Beam Pipe

• Gröbner multipacting parameter
• Horz 1.28 Vert 8.04 _at_30e10/bunch
• Energy required for electrons to traverse beam
  chamber in one RF period
• Horz 120 eV Vert 19 eV
• Electron energy gain at the extremities of the
  ellipse (impulse aproximation)
• Horz 2.3 eV Vert 7.3 KeV
• Maximum beam kick (finite bunch length)
• 772 eV
• Larmor radius 0.47 mm
• _at_ 2 KGs

(From Miguel Furman)
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Elliptical Beam Pipe
With normal MI elliptical vacuum chamber and
within bend magnets, at proton bunch intensity of
6e10, the electron cloud threshold for the bunch
length of 0.54m, which means electron cloud
happens during ramping and transition crossing
where bunch length becomes shorter
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Elliptical Beam PipeWith Clearing Electrode
Above electron cloud can be suppressed by the
500V clearing electrode in the beam pipe.
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6 Beam Pipe
For the 6 beam pipe, the electron cloud happens
even at bunch intensity of 10e10 proton/bunch at
low SEY1.3
The electron cloud can be suppressed by 50Gs
solenoid or over 500V clearing electrode
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Future Plan

• Continue the detailed simulation for various beam
  and surface conditions
• Beam studies at Tevatron and MI
• Electron density vs. beam intensity.
• Electron energy spectrum.
• Bunch by bunch tunes, loss, emittances.
• Vacuum changes.
• Comparing and benchmarking the simulation codes
• Test of mitigation methods with the test beam
  pipe Solenoid, clearing electrodes, coating,
  grooving, etc.
• Does it exists in Booster?

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Summary

• Electron cloud effect is a limiting factor to the
  high energy, high intensity accelerator
  performance.
• It might have some impacts on magnet design.
• The simulations and initial observations show the
  electron cloud will be a problem for SNuMi and
  future Fermi neutrino programs.
• More studies and investments should be put into
  this researches.